1. Field of the Invention
The present invention relates to a plasma processing apparatus.
2. Description of the Related Art
In applying, for example, an etching process to a semiconductor wafer by generating a plasma, a sheath voltage V.sub.S, which is greatly dependent on the energy of the ion and is an important parameter of the etching, is determined by a plasma potential V.sub.p and a negative potential V.sub.DC with which a negative electrode is biased. Since the negative potential V.sub.DC is more dominant than the plasma potential V.sub.p, it is important to measure particularly the negative voltage V.sub.DC.
Let us describe how the negative voltage V.sub.DC is measured in the conventional technique with reference to FIG. 1 showing a general RIE type plasma etching apparatus 101. As shown in the drawing, an upper electrode 103 and a lower electrode 104 are arranged to face each other within a processing chamber 102. A semiconductor wafer 108 is electrostatically chucked by an electrostatic chuck 107 mounted on the upper surface of the lower electrode 104. Under this condition, a plasma-forming gas is introduced into the processing chamber 102 through a gas passageway 114 and a plurality of gas spurting ports 115 formed in the upper electrode 103. At the same time, a high frequency power is supplied from a high frequency power supply 105 to the lower electrode 104 through a matching circuit 113 and a blocking capacitor 106 so as to generate a plasma in the space between the upper electrode 103 and the lower electrode 104, with the result that a plasma etching is applied to the semiconductor wafer 108.
For monitoring the negative potential V.sub.DC in the plasma etching apparatus 101 of the construction described above, an appropriate lead wire 110 is connected to a power supply wire 109 extending between the lower electrode 104 and the blocking capacitor 106. As shown in FIG. 1, the lead wire 110 is connected to the power supply wire 109 outside of the processing chamber 102. Further, a V.sub.DC monitor 112 is connected to the lead wire 110 with an RF filter 111 interposed therebetween so as to permit the monitor 112 to monitor the negative potential V.sub.DC.
It should be noted, however, that an anodic oxidation treatment is applied to the surface of the lower electrode 104, with the result that an anodic oxide film 104a is formed on the surface of the lower electrode 104. Further, the electrostatic chuck 107 is formed of an insulating material. It follows that it is impossible to monitor accurately the negative potential V.sub.DC even if the lead wire 110 is directly connected to the lower electrode 104.
In the conventional apparatus, however, it is unavoidable to connect the V.sub.DC monitor 112 to the power supply wire 109 via the RF filter 111, as described previously, though it is impossible in this case to monitor accurately the negative potential V.sub.DC. The value obtained by the V.sub.DC monitor 112 thus connected is used as the negative potential V.sub.DC on the semiconductor wafer 108. In this case, however, the expected result of the plasma processing cannot be obtained in some cases even if the frequency and output of the high frequency power supply 105 are controlled on the basis of the V.sub.DC level monitored by the V.sub.DC monitor 112 because an accurate V.sub.DC level cannot be obtained in the prior art as pointed out above.
On the other hand, where a plasma processing such as an etching is applied to a semiconductor wafer, occurrence of an abnormal discharge within the processing chamber tends to do damage to not only the semiconductor wafer but also the various devices and parts within the processing chamber. In such a case, it is necessary to locate the position where the abnormal discharge has taken place so as to take countermeasures promptly such as replacement or repair of the damaged parts of the processing chamber.
In the conventional technique, the abnormal discharge occurring within the processing chamber is detected by measuring the negative potential V.sub.DC as described above. To be more specific, the abnormal discharge is detected by observing the negative potential V.sub.DC monitored by the V.sub.DC monitor 112 shown in FIG. 1.
Where the negative potential V.sub.DC with which the lower electrode 104 is biased is measured by the conventional method described above, it is certainly possible to detect with some accuracy the abnormal discharge within the processing chamber 102 on the basis of the fluctuation in the detected V.sub.DC level. However, the V.sub.DC value monitored by the V.sub.DC monitor 112 is not necessarily accurate as described previously. In addition, it is not reasonable to determine that the fluctuation in the detected V.sub.DC value has been caused without fail by the abnormal discharge occurring within the processing chamber 102. For example, it is possible for an abnormality occurring in the power supply system of the high frequency power supply 105 to have caused the fluctuation in the detected V.sub.DC value. What should also be noted is that, even if occurrence of an abnormal discharge has been detected, it is impossible to detect the specific position within the processing chamber 102 where the abnormal discharge has taken place.
Further, in the case of mounting a semiconductor wafer as an object to be processed on the surface of an electrostatic chuck, the wafer is deviated from a desired location on the surface of the electrostatic chuck in some cases. If a plasma processing is applied to the wafer deviant from the desired location, it is impossible to apply the plasma processing uniformly to the wafer. In addition, the electrostatic chuck is exposed to the plasma, leading to insulation breakdown of the electrostatic chuck. Further, the wafer is likely to be blown away by the back pressure of a helium gas supplied through the electrostatic chuck toward the wafer for the heat transfer. The wafer thus blown away by the back pressure of the helium gas tends to be broken.